"Avoid Highways" ineffective when using "Alternative Routing (beta)"

🚧 THIS IS A TICKET MIGRATED FROM UPVOTY. The original ticket and comments will be available for a limited time over at:
https://abrp.upvoty.com/b/report-a-bug/avoid-highways-ineffective-when-using-alternative-routing-beta/
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When using this plan https://abetterrouteplanner.com/?plan_uuid=a445780b-7190-4d97-bea7-92c4339716f6 with "Alternative Routing" ABTP tries to use the A1 while I would expect it to use the B75 when enabling avoidance of highways, as it does with normal routing.