I am currently testing eM Client for our company in conjunction with Grommunio groupware, which supports all standard protocols and is used with Exchange MAPI, EWS, IMAP, etc. eM is connected with EWS.
We also handle importable calendars, such as holidays, trash dates, etc. We have noticed that some of these events are not displayed in the eM Client desktop or Android versions. If a repair is initiated, the events appear for a few seconds and then disappear. This problem does not occur e.g. with MS Outlook or the Android app Nine etc.