Causes for a burned up flyback diode on a relay?
Figure 1. Relay datasheet extract.

Checking the relay datasheet we find that the coil current should be about 33 mA. At the instant of switch-off the coil inductance will maintain that 33 mA through the flyback diodes.



Figure 2. Extract from the Vishay 1N4148 datasheet.

Checking the Vishay (chosen at random) datasheet we see that 33 mA is well within specification.

I would conclude that the designers did their job OK but that the purchasing department may have found some bargain diodes.



Update for "T4" diodes:

The Diodes Incorporated datasheet rates their diode at 250 mA continuous. That isn't the problem either.