[2.0.20] Trains blocking at stacker entrance instead of re-pathing into newly opened lanes

[copperpot]

In a stacker-based depot that doesn't quite have enough stacker lanes to accommodate all trains simultaneously in a worse-case traffic jam, I'm getting backups at the filter chain signal (double-parking prevention) because trains blocked at the filter chain signal are failing to re-path into newly opened stacker lanes.

In these two screenshots, an inbound Iron Ore train decided to path through the topmost stacker lane (all stacker lanes were occupied when it pulled into the depot), but it just continued to sit at the filter chain signal even after two lanes had opened up (that were each two blocks closer to the target train station):

427520_17.jpg (1.04 MiB) Viewed 152 times

Once the ore train already occupying the target stop departed the block, the inbound Iron Ore train immediately re-pathed and used one of the two open lanes to pull into the station:

427520_18.jpg (935.78 KiB) Viewed 152 times

What gives? Why aren' t my backed-up trains always re-pathing to pull into newly open stacker lanes, and hence minimizing the number of blocks between them and their destinations?