You can no longer change multiple exercises in strength activities.
After selecting the exercises you want to change, the drop down list appears behind the edit box, making it unusable.

I emailed Garmin Product Support about the same issue today.
CRB, after submitting a video showing the issue, I received a response from Garmin Support:
Thank you for that information.…
I emailed Garmin Product Support about the same issue today. For now, the workaround is to edit exercises individually or use the mobile app.
I also noticed another thread mentioning multiple issues, including…
Thanks for your post, 7542165 . I've got this logged and it is being looked into. For others who may be experiencing this same overlapping, please bookmark and follow this thread as we will post any meaningful…
I emailed Garmin Product Support about the same issue today. For now, the workaround is to edit exercises individually or use the mobile app.
I also noticed another thread mentioning multiple issues, including this one - Bugs in Calendar & Activity View. As far as I can tell, no one has reported it to Garmin Support yet.
I emailed Garmin Product Support about the same issue today.
CRB, after submitting a video showing the issue, I received a response from Garmin Support:
Thank you for that information. I do see that our team is investigating this issue further at this time and I have added you to this report. We will be reaching out if any additional information is needed.
They just fixed it. It was still an issue when I went to add my exercises from my morning workout right after I posted.