I do not think anything changed between Gimp versions.
You can try changing settings in
Edit -> Preferences -> Interface -> Image Windows where Initial zoom ratio can be either
Show entire image or
1:1 see: Gimp docs
https://tinyurl.com/5n84x4ym for what use that is
AFAIK the default is
Show entire image If smaller than the window you get 100% size. Larger than window then less than 100%
With 1:1 it will always be 100% No scaling up for a small image, and no scaling down for a large image.
Quote: They open at seemingly random scales, sometimes tiny on screen and sometimes massively zoomed in.
Possibility you have 1:1 enabled.
edit: Ah..just a thought. Do you by any chance have
View -> Dot-fot-Dot toggled off. That will display images at print size and that depends on the print resolution. The same pixel size image at 72ppi displays larger than at 300 ppi.
I do know the annoyance with smaller images, I have re-assigned ctrl - shift - j to a single key.
There are also the number keys for fixed size zooms.