Can someone explain what's going on? Has routine/planned maintenance been suspended? Seems to be an increasingly common phenomenon (alongside daily crew shortages)

As always (it seems) numerous Reading-Gatwick services cancelled for the same reason

Cancellations to services between Liskeard and Looe

Due to more trains than usual needing repairs at the same time between Liskeard and Looe all lines are blocked.
Train services running to and from these stations will be cancelled. Disruption is expected until the end of the day.

Customer Advice
The line between Liskeard to Looe is suspended until further notice. This is due to more trains than usual needing repair.

Cancellations to services between Plymouth and Gunnislake

Due to more trains than usual needing repairs at the same time between Plymouth and Gunnislake all lines are blocked.

Train services running to and from these stations will be cancelled. Disruption is expected until the end of the day.

Due to more trains than usual needing repairs, the lines between Plymouth to Gunnislake ( both ways) will be suspended until further notice

 
Less 150s seem to be available. There are up to 5 turbos now operating on the Devon Metro.

Maybe all the 150s elsewhere being scrapped need cannibalising.

 
I’ll take a guess at air con (or air cooling) failures on an already extremely stretched fleet. 

 
UPDATE: Both lines are now open.

 
Of course neither line was blocked - quite the opposite.  There were no trains available to run on them - so they were totally clear.