I don't know what the expected behaviour of the Daily Reward system is, so I don't know if any of the things I have observed are unexpected, or bugs, or not. The system does seem inconsistent in its behaviour if you delay claiming a reward.
April
25th, logged in, reward is 10gp, collected the reward.

26th, logged in, reward is 2 clothing research scrolls, collected the reward.
27th, logged in, reward is 10 Cr Repair Kits,
did not collect the reward.
28th, logged in. I collected the 10 Cr Repair Kits due from the 27th. At that point the reward dialogue said the next reward was not available for 13h, at midnight UTC. But, I straight away logged in to another character and immediately received a notification that the next login reward was available to be claimed - the 5 Green Riding Lessons for the 28th. I collected that reward.
29th, logged in, reward is 50gp, collected the reward.
30th, logged in, reward is 8 Crown Blacksmithing Research scrolls.
Did not collect the reward.
May - reward system resets. 
1st, logged in. The reward dialogue showed 50gp reward available to be claimed. I collected the reward. The dialogue then showed no reward to claim until midnight UTC. I logged in to another character, and it still showed no reward available until midnight UTC. It looks like the opportunity to claim the 8 Cr Blacksmithing Research scrolls from 30th April has gone.
2nd, logged in, reward is 80 Crown Lethal poisons, collected reward.
3rd through 6th, logged in each day, reward offered is 5 Stamina Riding lessons, did not collect reward.
7th, logged in, claimed 5 Stamina Riding lessons, UI now shows 16h to claim 10 Tri-pots, immediately logged in to another character. Able to claim 10 Tri pots, UI then shows 5 Speed Riding Lessons available in 16h, immediately logged in to another character. UI still shows 5 Speed Riding Lessons available in 16h.
So it looks like you can only catch up one day of ignored rewards, wait two days and you lose the credit. I lost the login rewards for the 5th, 6th and 7th.