I have been having this problem for years. But really bad in the last year.
over a series of say 20-35 projects, my brush loses opacity. It makes it so that when I am using the brush to clean up a mask I have to click like 15x in the same spot to get a fully opaque mask.
I use to reset it but that only gets me to a point where I only have to click like 6x to make it fully opaque. Now, in my flyout menu in brush settings, there is no reset brush option.
In this picture, you see a semi-opaque circle on the teeth. This is 4 clicks to get it to that point.
There is no reset in the flyout.
It has been happening like this for about 3 years total but no update has fixed this nor have I found and reset option that will fix the problem. It is not like that after a fresh install. It gradually gets like that every time. Plus I do not want to have to reinstall PS every 30 projects.
Do the actual settings alter as the brush loses opacity? That would mainly be in the Options bar, as Jane has said, and both opacity and flow could cause your problem.
Do those setting change from 100% over time?
Are the icons for opacity and flow turned on? (my green highlight)
Also check Brush Settings > Transfer. The only setting that would change over time is if jitter is set to Fade with a high value. 9999 is the highest value allowed, and the brush would drop from 100% opacity/fade to zero after 9999 stamps of the brush tip (as controlled by brush spacing). I am 90% sure that Fade resets each time you use the brush, but you do have a problem so we do need to look at all possibilities.
From your screen shot it looks like you are using a default hard round preset. Is that the case, or are you using other presets? This could apply if you are using third party presets, so look through the Brush Settings to see if anything is turned on.
Erodable tips came to mind, but as far as I know, they only work with things like tip sharpness, size and softness. I am 90% sure that opacity can not be used with erodable tips.
My absolute first thought was are you using a tablet, and if so which one? If you told us it was a Chinese tablet like Huion or Chuwi then I'd be looking at drivers or inherent problems, but I can't see that being the case if reinstalling Photoshop fixes your problem for a while.
Another possibility is a partial selection. Does Ctrl D restore apparent opacity? Are there any masks involved?
The fact that the issue has remained with multiple Photoshop updates and reinstalls means you need to look for a common denominator, and something that is introduced after a reinstall (like custom brush presets). Have a think about that and answer our questions and we'll see if we can track your issue down.