Issue Setting Domains AllowList for Sharing in CSOM API

If you want to switch between block and allow mode, you need to set the mode to None, then execute and the set it to whatever your looking to do.

var siteProperties = tenant.GetSitePropertiesByUrl(siteCollectionUrl, false);
ctx.Load(siteProperties, s => s.SharingDomainRestrictionMode, s => s.SharingAllowedDomainList);
ctx.ExecuteQueryRetry();

siteProperties.SharingDomainRestrictionMode = SharingDomainRestrictionModes.None;
siteProperties.Update();
ctx.ExecuteQueryRetry();

siteProperties.SharingDomainRestrictionMode = SharingDomainRestrictionModes.AllowList;
siteProperties.SharingAllowedDomainList = domainString;
siteProperties.Update();
ctx.ExecuteQueryRetry();