A lot of people make the mistake of thinking that endurance is the way it's not.

If you have done a proper high intensity work out you should feel fatigued which is why you then need to rest so you end up only working out 3 times a week.

Have you ever tried out a weight training exercise you haven't done before – so you pick a weight to start...  One rep, you think, "Oh this is easy.  I may need a heavier weight."  After three reps, you start feeling it.  And after six reps, you literally cannot do another one?

Is this normal (especially for underworked muscles)?  Or could it be a sign of too much caloric deficit or something?

I haven't had that particular experience but it's muscle fatigue, not caloric deficit.

https://en.wikipedia.org/wiki/Muscle_fatigue