The reason is:
Per design Field Proficiency Caches are being rewarded to agents after having reached Season Level 100. While climbing the Season Level from 1 to 100 you will receive the rewards from the Season Reward Track(s). And after having reached level 100 you will receive the caches again. As long as the Season is active.
Btw:
This has always been like that ever since the Field Proficiency Caches were reintroduced back in Season 2:
We are also reintroducing Field Proficiency Caches with Season 2. After reaching Season Level 100 every time players’ SHD Level increases they will receive a Field Proficiency Cache!
Source: Ubisoft News, June 2020
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