It does not seem to be possible to un-set the "mtu-tun" option from the client. Entering an empty value as MTU leaves "mtu-tun" without a value in the config file, which breaks the configuration. Since mtu-tun needs to be set both sides, this may eventually cause problems with servers which do *not* have mtu-tun set.
Crap abandonware
Fixed: If the field is empty no "tun-mtu" will be written to the config file.