Sentry mode oddity

edit: NM I found that you cannot Sentry a ship that is below half Supply, regardless of the presence of a Depot or not.

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I have been dabbling with Asteroid Miner ships, and to keep end turn orders simple I park the miners on Sentry orders once they reach the asteroid field. This affects both Balance Mod and normal games.

Sometimes I find I cannot put the miners on Sentry orders. Ships before and after in the turns queue can be sentried as expected, but once a while the Sentry order stays greyed out. From that point on it will refuse all attempts to Sentry it.

At first I thought that there were enemy vessels nearby to disturb the Sentry, but nothing I can see on my sensors, and if I cannot see it, it should disturb my ship, right?
I have tried moving the affected ship elsewhere, but that doesn't seem to help.
I have tried using the Mining Minister, but the ship continues to come up every turn as elegible to move.

Small Freighter (+2 size bumps) Only used in systems with a Resupply base
Bridge+CrewQ+LS
Basic Sensors
1ea. Robo-Farmer/Miner/Refiner
2x Engines

Out system version adds a Solar Panel and later a cargo bay(may take Natural Merchants to avoid needing a cargo bay)