What does this doted speech bubble mean
iPhone 12 Pro, iOS 15
In last 11 years of my association with Apple devices I have never come across this "doted speech bubble" in an iMessage.
My guess is...
You sent a text message initially (Green Bubble) indicating your device was not connected to the internet. The immediately you sent an iMessage to the same person.
Dotted outline around a regular text bubble: This could indicate a message that wasn't synced properly between your devices.
If the message content or attachments aren't appearing, syncing your iCloud account might help.