I never use this setting, but was curious. And yes, upvoted, its not working as expected.
PR Beta 25.6.0.14. Win11. Note that I frequently reset the cache for testing, and it is under 2 MB at the moement.
I changed the preferences setting from my current "do not delete" to "when cache exceeds," leaving the default of 45GB. I exited and entered preferences, and it remains 45 GB.
If I click outside the entry box, it shows the number I entered; then I click OK.
I changed it to 10GB, and on entering, it shows 453GB.
I change to 45GB, it shows 1GB.
I tried 45 again, it shows 1.
Change to 10, shows 453.
And finally, I changed back to "never," which works. Then back to cache exceeds 45, and it shows 1.