I think you may be right about this. I don't know why they don't gain the XP when they should have after they defeated enemies in combat.
Using Training Tomes for Companions to gain Experience & tasking them to do Brawlin' for XP are two ways right now, even though it provides them some Experience, but gaining XP from battling enemies should be another way for Companions.
I think them battling enemies to gain Experience should be considered available to Companions, too, rather than just us Pirates. Why should we have all the credit for that when they should share it with us, too?
Anyway, after all the battling they did right from the start of their adventures to today, I say they really should deserve it.
But 500 xp out of 6000-ish at level 50 by waiting 8-ish hours, is definitely not enough to keep your companions leveled
This is what I did for my son's pirate to help his companions max faster. His pirate was maxed & he only had 2 rows of level 65 or higher companions. He also had 1/2 a page of lvl 55-64 companions, 1 1/2 pages of companions in the 40's to lower 50's range, a page barely into the 30's & 1 page of companions level 20 or lower. I made sure about 3 rows were maxed right away, set up the rest of the lvl 55+ companions on the 1st page with Brawlin' & let others concentrate on Pet Wranglin', Scavengin' & Plunderin'. In my opionion it's not really worth assigning any companion lower than level 50 to Brawlin' because, as you stated, it takes far too long to bring in too little XP. The higher level your companions are, the less turns Brawlin' it takes to level up that companion.
So for that reason, upon the 55+ Brawlin' designated companions maxing, I took 5-10 of them & placed them on Keel Haulin' to bring in training tomes. Instead of assigning a level 30 companion to Brawlin' & taking quite some time to level up, I would use the training tomes that were brought in to rapidly increase the companions level- due to the huge difference between the companion & pirate level. So instead of assigning a level 30 companion to Brawlin', & it taking a few days just to move up level, I would take 1 tome that took around 8 hours to bring in, & use it to move that level 30 up 4 or 5 levels. Meanwhile that companion was still bringing in gold, some goods to sell or pet training supplies. As the tomes come in, you train 1 companion until it gets to around 60 (it will still be gaining a level or two per tome until then). Once it hit 60 I would switch that companion to Brawlin' since it would be bringing in over (I think) 1200 XP or so-- at least over 1000. So it would now be able to level on it's own faster.
Then you concentrate on another companion or 2 & keep repeating until they are all up to at least level 60. If you desire, you can continue to bring in tomes & train them until they hit max, but it takes up to a few tomes per level when you get close to 65. The XP brought in from Brawlin' at those higher levels, acts like a training tome by taking just a few more turns per level to increase the companions level. Every time a companion maxed, I would then assign it to another task like Pet Wranglin' to bring in some higher level snacks for my pets.
But by focusing on training tomes & training your companions up to a certain point before switching to Brawlin', it will take far less time then just assigning those same lower level companions to Brawlin'. At the lower levels, when your pirate is maxed, you can jump them up 3, 4 or even 5+ levels at a time in as low as 7-8 hours. Brawlin' at those same levels will take possibly days just to increase them 1 level. So, for me getting them to at least 55, preferably 60, since at 55, with your pirate maxed, they can still move up 2-3 levels from 1 tome, before concentrating on Brawlin' saved soooo much time.