Sound like it would be better to use a Slimmed down version of Microsoft-Windows-EditionSpecific-EnterpriseS-Package to make an edition switch package, because edition switch is needed to update Server 20348.xxx as Client will not update.
@Skylined Wihich version of Microsoft-Windows-EditionSpecific-EnterpriseS-Package did you use to add IoTEnterpriseS to 20348.681.amd64.fe_release_IoT_Enterprise_ltsc_en-US_2.iso ?
For 25398.1, Microsoft.SecHealthUI_8wekyb3d8bbwe removes itself after oobe. I can open it during oobe by shift+f10->explorer->shell:appsfolder, but it removes itself after finishing the oobe. How to solve it? I'm using 1000.25398.1.0 version
Tested 25398.1551 Enterprise update integration with custom servicing stack Spoiler For 25398.1, /resetbase doesn't delete outdated ondemand packages. Q: How to cleanup 25398.xxxx correctly?
There are Windows Server build 25398.1.iso\sources\install.wim\1\Windows\servicing\ FodMetadata InboxFodMetadataCache
I think it'd be better to use "Client License and Key" only. Because with "full Client Barebone System without Client language pack", same outcome.
I haven't checked, but couldn't it be the same reason as in the case of 26100.1? I think it may require a baseline update to make a successful resetbase. But it's just my guess, not sure it could be the solution or not.
It will not work with 20348.1 Pro, as it is not defined in update.mum (no Client editions are in fact defined there). Your only bet is to install both Client and Server packages at once, update as Server, and then go back to Client.