Without knowing which version you are using, it's hard to test but I just went back and pulled up a 2015.4 and 2020.2 file and was able to change the USD rate in Setup, Settings, Company, Currency, Exchange Rate rate in multi-user mode when logged in as sysadmin.

I then went to Sales Invoices and picked a USD customer and the new rate I entered showed up.

I then moved the data back three weeks and the rate applicable at that time was in the Exchange Rate field.

When I change the rate to something different when the above settings have a specific date for a different rate, I get the image below and when I say Yes, the Settings rate changes as it should.

Are you saying this is what you are seeing and it is not normal for your file?  This is how it has worked all the time I've been working with the program.

No.  Table rates seem to revert back to a previous rate & unknown why.  I don't work with the software daily, but noticed something odd with Gain & Loss so went to check movement in rates.  That is when I saw rates duplicated month to month!  Went to look at Unrealized Gain & Loss reports and the correct rates were shown in the reports; many early transactions in the month had the correct rates (from the table) and somewhere along the way, the rate reverted to the previous month rate!  So we 'tested' and in the table, set up August rate to 1.3042; the next day before anyone had done anything, the table showed the rate at 1.3404 - reverting back to July's rate!

Before:

: 

After: 

Using Sage 50 Premium 2020.0