The same thing is happening to me as well. For example, at the beginning of this month, the total rainfall counter was at 647.8. We had 19.4 mm on October 1, and the rainfall counter at the end of the day was 667.2 as you'd expect, however, at midnight, the 19.4 mm didn't reset to 0, so it added another 19.4 mm making the counter jump up to 686.6.
This basically happens every single day, so as a result, the monthly rainfall for this month is currently at 174.4 mm despite the fact we're only 7 days into the month.
What's weird though, is that at the time Cumulus MX is actually launched, the total rainfall counter is decreased by the same amount. For example, it went from 694.2 to 674.8 (a decrease of 19.4 mm equal to the previous day's rain), which means the total rainfall seems to suddenly increase by the previous day's rain from midnight until the time you launch but is then reset to what it should be. In terms of the total rainfall counter, this isn't too much of a problem, however, if more rain falls on one day than the previous day,
before Cumulus is launched, then it subtracts the previous day's total at the time Cumulus is launched. As an example, we had 3.8 mm one day. Then, the next day, by the time Cumulus was launched, we had 28.8 mm, but then the Oct21log.txt file suddenly showed that it dropped from 28.8 mm to 25.0 mm (which obviously is exactly 3.8 mm difference). We had another 0.2 mm of rain later, which meant the total rain for that day was recorded as 25.2 mm, when it was actually 29.0. I have attached an image of the graph showing the 3.8 mm from the previous day, the 28.8 mm up to just before 14:00 and then a sudden drop by 3.8 mm to 25.0:
This whole thing then affects the total monthly rain, as it will add the previous rain to the next rain, but will also subtract the previous rain from the daily rain total.
The actual daily rainfall totals should be
1st - 19.4
2nd - 38.6
3rd - 0.2
4th - 3.8
5th - 29.0
6th - 0.0
totalling 91.0 for this month so far.
However, what they are recorded as in dayfile.txt
1st - 19.4
2nd - 38.6
3rd - 0.2
4th - 3.8
5th - 25.2 (equal to the actual total, minus the total on the 4th, which only happens if the amount of rain is higher than the previous day, before cumulus is launched, otherwise, it remains the same)
6th - 0.0
totalling 87.2 for this month so far.
Additionally, Cumulus MX is creating duplicate dayfile.txt entries. One of them is showing actual times for any records, e.g. Max temperature time, but the other one shows just 00:00 for everything. As a result, not only is the monthly rain total wrong because of the daily rain totals, it actually doubles the wrong amount so it would actually say 174.4 mm instead of 87.2 mm (both of which are wrong, since it should be 91.0)
This part can be fixed by deleting every entry that has a duplicate and then using CreateMissing.exe to recreate them. Normally I hibernate my PC (with Cumulus running), then when I wake my PC up, I'll then close Cumulus, copy the 'data' folder contents from the midnight backup, then run Cumulus, so it creates the missing data during the time the PC is hibernated. Normally, and for years, this has never caused a problem, so I'm going to assume that this is not what is causing the dayfile.txt entries to be duplicated.
I can't attach any MXdiag logs since the log for the last day that any rain was recorded, is too large (6404 KB) - this also means I can't "pastebin" it either.
However, here's some of the entries in the MXdiag log that mentions anything to do with "rain" if this helps.
2021-10-05 13:42:23.607 ReadTodayfile: Rain day start = 710
2021-10-05 13:42:23.607 ReadTodayfile: set initialiseRainCounterOnFirstData false
2021-10-05 13:42:23.611 G:\Desktop\CumulusMXDist3148\CumulusMX\data\alltime.ini
2021-10-05 13:42:23.611 Alltime.ini file read
2021-10-05 13:42:23.616 MonthlyAlltime.ini file read
2021-10-05 13:42:23.619 Month.ini file read
2021-10-05 13:42:23.622 Year.ini file read
2021-10-05 13:42:23.623 LoadDayFile: Attempting to load the day file
2021-10-05 13:42:23.713 LoadDayFile: Loaded 2557 entries to recent daily data list
2021-10-05 13:42:23.715 Finding raintoday from logfile data\Oct21log.txt
2021-10-05 13:42:23.715 Expecting listsep=, decimal=.
2021-10-05 13:42:23.747 Midnight rain found in the following entry:
2021-10-05 13:42:23.747 04/10/21,00:00,8.5,95,7.7,0,0,0,0.0,0.2,1021.1,706.2,22.2,70,0,8.5,8.5,0.0,0,0.00,0.00,8.0,0,0.0,360,0.0,0.0,8.5,8.5
2021-10-05 13:42:23.756 Midnight rain found, setting midnight rain count = 706.2
2021-10-05 13:42:23.756 Checking rain counter = 713.8
2021-10-05 13:42:23.756 Rain counter set to = 713.8
2021-10-05 13:42:23.757 Getting rain totals, rain season start = 1
2021-10-05 13:42:23.757 Today = 05/10/21
2021-10-05 13:42:23.758 Rainthismonth from dayfile: 120.2
2021-10-05 13:42:23.758 Rainthisyear from dayfile: 316.8