The UK site (https://www.pioneerdj.com/en-gb/product/all-in-one-system/archive/xdj-rx/black/support/) does show that 2.21 is available for the XDJ-RX (Pioneer, this is a real problem on the US site, right now there is no way I can find at all to get any firmware for the XDJ-RX at all), though none of the release notes seem related to your issue:
Ver.2.21 (26 October, 2017 Update) [FIXED] With SYNC enabled certain MP3 files did not sync. Waveforms were not shown when tracks were imported to some USB hard disc drives. Auto BPM did not work when selecting MASTER for BEAT EFFCT channel while selecting LINE/PHONO for Input. [IMPROVED] Beat countdown MEMORY CUE color specification changed to match other cue points. Accuracy of the tempo slider when the XDJ-RX was used as a controller with rekordbox dj.