PHP FCGId sub-processes option disappears after setting it to "None"

If I set the field “PHP FCGId sub-processes” to “None” (under Virtualmin - Server configuration - PHP Options) it disappears and I can’t set it back to any number.

Is this a bug? How can I set a number of sub-processes again?

Thanks in advance!

SYSTEM INFORMATION
OS type and version: Ubuntu Server 20.04.03
Webmin version: 1.981
Virtualmin version: 6.17-3
Related products version: PHP 7.4.3