The zimbra logs show that the devices keep trying to sign in in some way but are presenting an empty password to Zimbra (see log below). The rate of the failed login attempts also seems to vary.
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2021-09-28 08:14:34,156 INFO [qtp2076287037-995728:https://webmail.affecteddomain.com/service/soap] [name=affecteduser@affecteddomain.com;oip=<users home IP address>;port=44328;soapId=cb411e6;] SoapEngine - handler exception: authentication failed for [affecteduser@affecteddomain.com], empty password
2021-09-28 08:14:44,219 INFO [qtp2076287037-995722:https://webmail.affecteddomain.com/service/soap] [name=affecteduser@affecteddomain.com;oip=<users home IP address>;port=44596;soapId=cb41220;] SoapEngine - handler exception: authentication failed for [affecteduser@affecteddomain.com], empty password
2021-09-28 08:14:54,356 INFO [qtp2076287037-995835:https://webmail.affecteddomain.com/service/soap] [name=affecteduser@affecteddomain.com;oip=<users home IP address>;port=44822;soapId=cb4124f;] SoapEngine - handler exception: authentication failed for [affecteduser@affecteddomain.com], empty password
2021-09-28 08:15:04,415 INFO [qtp2076287037-995826:https://webmail.affecteddomain.com/service/soap] [name=affecteduser@affecteddomain.com;oip=<users home IP address>;port=45094;soapId=cb41287;] SoapEngine - handler exception: authentication failed for [affecteduser@affecteddomain.com], empty password
2021-09-28 08:15:14,481 INFO [qtp2076287037-995824:https://webmail.affecteddomain.com/service/soap] [name=affecteduser@affecteddomain.com;oip=<users home IP address>;port=45366;soapId=cb412bc;] SoapEngine - handler exception: authentication failed for [affecteduser@affecteddomain.com], empty password