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The G-suite documentation[1] provides this information, but I have submitted a feature request to make it clear on the Cloud DNS public documentation and the Cloud Console. However, I can't provide you with an ETA or guarantee the implementation of this feature, but rest assured that Google strives on improving its products and that your feedback helps us do just that.

Any future updates to this feature will be posted here.

[1] https://support.google.com/a/answer/173535

Any updates on this?

Ditto. This is a frustrating one.

This hit me too. At the very least, the error message returned by the API could explain what is wrong: that the string can't be longer than 255 characters.

Cloud DNS upon emitting multiple TXT lines seems to re-order them. Even if you managed to enter a TXT record using multiple lines, this will render the entry unusable.

For example, this won't be recognized as a valid DKIM entry:
"DNDIed6Ys/R3J2QzhK/sEnJ+K/K4uD1yIuvK/6tzcgGcSKKFJ9Ffr/1x44eXjLXsfy9n8evvljnWwFe/U2tAtUuK7olU3CXWZGbNir5gwBbU8gE1UuVLqeW8xJgk+fj6+SMTXejvGpvcy4dsHtHNLSwIDAQAB"
"v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAjOLvrcvHRt8Uzqmq94BEYYyuqziuMcQCG7xIMdm/AMOmpwec6xI0craHP/9g4pfdpgiTzFgYXaAu/Y9bKomEj1dRsmvSkOEX17p89h0w5yLxEru7MsLhdVJ26JvnQeSWY/aet5nOqhH15DrjTfETGenHYDXt/sE80sfkkDdyRUuGZyC92odpMaaE6fwkxXa"

… which makes it quite severe.

Same happened to me, the entries are reordered and unsable:

text = "5IL88bqDLXC3TLufrcfR4TztNLb5gSGWIPK6adGCI/sppQYiF+D1lCNtefspmFoWps8j2QpRWOyLdOMhn56jjsiew8hSDz"
text = "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAlySp/8voQMhWUc2KxPzGs71ZxdS2y3DfCG1MOW52G7iQXnwXaERk127YEAgeS6KYTc14u+"
text = "TmW4/ZuYfvToBMSlLkg40JLtoLQtlP2QBoPm9VEkTBLDv84u2Bsp8ycSe02WVxscgoMsyfM0WIiD+cBlYJKFvAX82bCZgdILVfM7nzxJ9EFbdKDusRU+Wf3dSRp9TGp/i5opckxVR47UJn0mvkmzU5wHtyVjQSBnpERcmWvqHg+f7HYgxwIDAQAB"