[Form] Fixed: String validation groups are never interpreted as callbacks
merged branch bschussek/issue7371 (PR #7918)

This PR was merged into the 2.1 branch.

Discussion
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[Form] Fixed: String validation groups are never interpreted as callbacks

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7371
| License       | MIT
| Doc PR        | -

Commits
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7b2ebbf [Form] Fixed: String validation groups are never interpreted as callbacks