A Blueprint Interface function that returns a map type variable will fail compiling with various errors when called in another Blueprint. If the same function is setup in the Actor Blueprint instead of the Blueprint Interface it will compile with no issues.
This issue was reported and tested in 4.21.2 (CL-4753647). This was reproduced in 4.20.3 (CL-4369336), 4.22 (CL-5159901), and Main 4.23 (CL-5204310)
Results: LogBlueprint: Error: [Compiler NewBlueprint] The type of Variable is undetermined. Connect something to Get Map to imply a specific type.
LogBlueprint: Error: [Compiler NewBlueprint] The type of Value is undetermined. Connect something to Get Map to imply a specific type.
LogBlueprint: Error: [Compiler NewBlueprint] A variable needs to be connected to Variable
LogBlueprint: Error: [Compiler NewBlueprint] Default value '' for Value is invalid: 'Unsupported type Wildcard on pin Value'
Expected: Compiles successfully
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-70642 in the post.