Is this a bug or a plugin conflict?

I have noticed lately that whenever I’m trying to problem-solve a DIFFERENT issue, and I need to deactivate ALL plugins to figure out what the conflict is…I can deactivate everything by selecting the top checkbox….but after that, if I try to check groups of plugins to REactivate…it NEVER will do more than one. So I end up always having to do ONE.AT.A.TIME. This is particularly tedious when I figure out very early what the problem plugin is and I just need to get the rest all activated!

So WHY is it that I can no longer check more than one plugin to reactivate them???