RotatingFileHandler seems to write logs to the .1 file that is already rotated

I implemented the RotatingFileHandler as belolw.
According to this manual, The file being written to is always app.log.
However, in my case, the size of already rotated log file, which has “.1” appended to the end of its file name, increases and the content actually changes.
app.log increases, too.
Why is this happening?

logger = logging.getLogger(moduleName)

h_format = logging.Formatter(
    "%(asctime)0.19s - %(module)s [%(levelname)s] %(message)s "
fh = logging.handlers.RotatingFileHandler("/var/log/app.log", maxBytes=100000, backupCount=10)

is for a version of Python that reached end of life a decade ago. Doesn’t matter in this case, I think, but you should use the official documentation instead.

Do the files grow past the limit set by maxBytes?

Is the content that is written to the .1 file the same as is written to the actual log file, or is it different?

Are you perchance using multiprocessing?

There were several threads and the RotatingFileHandler was created by each thread.
When I modified as only one handler is created and shared by all the threads, the problem was gone.