As documented, the ADS converts all RMTES to UTF8 in JSON so that it can be consumed without requiring any clientside libraries. This includes the display strings for enumerated fields (so for fields like PRCTCK_1, possible values include an actual upward or downward arrow, e.g. "⇧"’). Content posted by client applications for RMTES fields should only contain ASCII characters as no conversion from UTF8 to RMTES is available.
This question was first asked in an Introduction to the Elektron WebSocket API webinar (29th of Nov). Please see here: Introduction to the Elektron WebSocket API