1. InDesign 18.3 but has been doing this for many years.

2. Draw a single column text frame next to a multicolumn text frame, have the same number of lines in both frames, apply Fit Frame to Content with any method to both frames, they will most likely not be the same height. You can change the width of the multicolumn frame and then Fit Frame to Content might get closer.

3. Should be the same height if all text styling is the same.

4. Variable height on the multicolumn frame, doing Fit Frame to Content a few times and can change it each time.

This is particularly frustrating when you cannot align the top of the frames due to layout, so instead, you have to zoom in, place a guide, and manually nudge so the text baselines line up.