Weng,
I have logged an enhancement request regarding this issue with using constraints in non-required fields in Survey123. The enhancement number is ENH-000099729. In the meantime, I've developed some workarounds that might suit your needs while we wait for an update:
Workaround #1 - Create another survey question prompting the user to designate whether s/he wishes to submit their email address by selecting 'Yes' or "No.' User could then designate the constrained email field to only appear if user selects 'Yes.' This can be accomplished by entering the following string into the "relevant" field of the email response survey question in Survey123 Connect. If the user selects 'No,' the optional constrained field does not appear and the user is able to submit form answers normally. An example of this workaround with an example of valid input:
Workaround #2 - Enter an example email such as "example@domain.com" as a default parameter in the default field of the optional constrained email survey question. If a user does not wish to log their email information, they can simply avoid the question entirely and leave the default. After responses are gathered, administrators could filter out the "example@domain.com" responses. An example of this workaround:
I hope this helps!
ichivite-esristaff