I just tested a few old APE files (tagged with MC14 and MC15) and a newly created file (tagged with MC17). MC18 had no difficulty in reading the Replay Gain tags. The JRiver programs have always used the same old proprietary "Replay Gain (radio)" and "Peak Level" tag fields inside the physical APE files.
However, if you have analyzed the files outside MC and some other application wrote the Replay Gain tags, the tag format is probably different and unfortunately MC cannot read the tags. Most other programs write the "REPLAYGAIN_TRACK_GAIN" and "REPLAYGAIN_TRACK_PEAK" tag fields.
A similar compatibility problem with the MP3, FLAC and Vorbis tags was fixed years ago. From time to time I have suggested fixing also the APE tagging system (Monkey's Audio, Wavpack, and Musepack use the APEv2 tag format).