When I successfully plant a live grenade and kill an enemy, sometimes i get xp for the kill, and sometimes I don't. Is there qualifiers to get the xp, or is this a bug?

I've had this. I don't think it's a glitch but rather more intentional. I'm waiting on other members of the fallout forum to provide the answer.

Doing something as fun as this doesn’t need any additional reward like XP or loot.

Are you killing civilians?

Raiders / Super mutants, so far. I don't kill anything that isn't a threat to me aka the innocent. Ps. I have had xp rewarded for sandman kills but doesn't happen every time.

I'd been wondering what happened to my exp alerts even shooting normal enemies normally. I assume I missed the message or it got combined with something on a double kill. Killed 2 super mutants once and it said 100 exp when I was getting maybe 45 each for others around there.

This is kind of refreshing for me. I personally don't like getting xp for every damn little thing. I watched a mirelurk kill a raider yesterday and I got xp for the kill. WTF?

I personally don't like to level up quickly. It robs me of the satisfaction when I do eventually level up. The first mod I'm going to get is a level slow mod to make levelling about three times slower.