Maximize LoPy message size

The maximum depends on the data rate. So I assume the data rate dropped to a rate too low to allow for the 150 bytes:

As for why the data rate changed: maybe the node did a new OTAA Join and only succeeded at some low rate (high SF), or maybe it used confirmed uplinks and failed to receive the confirmation, hence also lowered its data rate and tried again.

In short: change your application to not need the maximum.