I tested this in Hole I have a 50 on account and when i pulled a level 46 mob i got the (with a bonus) xp message and received 1.370 xp with a 25% xp pot going and full group. Our puller did not have a 50 on account and when he pulled a 46 mob i got no (with a bonus) message and received 1.254 xp
To confirm further just had our 48 Ranger that i was not getting xp bonus message on leave and a 50 Ranger repped, no one else leveled and xp on 54 golem with 48 ranger was 1.599 as i leveled to 47 and with 50 ranger that joined all else remaining same, i got 1.689 onnext 54 golem and got the bonus message
Yup, seeing this on my character with a 30% bonus. If I pull, I get the bonus. If someone without a 50 pulls, don't see any. not sure if you get your full bonus if a 50 pulls with a lower bonus though.
This is the primary draw of this server (Alts). It's disappointing we are so far in to classic, where this is the only bonus, and it's just horribly broken.