This is standard for Thunderbird releases and confirmed by Ikey who works for them. They hold back for existing users until they have a release they are content with and then "open the floodgates".
Example: 102 released 5th July 2022 and was not part of thunderbirds automatic updates (for Windows users) until 102.2.0 released 23rd August 2022.
115 blogpost explains this a bit more:
As with any major release, we sometimes become aware of corner cases after significant public testing. The Thunderbird team will wait to enable automatic updates until we’re confident no such issues exist. We’ll keep you posted right here, and on social media platforms. (Existing Thunderbird 102 users will continue to see security updates during this time).
On Linux, the timing of this release and subsequent automatic updates may be determined by your distribution’s maintainers. If it’s convenient for you, we recommend installing the Thunderbird Flatpak from Flathub, which is now packaged and maintained by the Thunderbird team. (Please note that 115 is not live on Flathub at time of publication, but as always we’ll keep you updated.)
Source: https://blog.thunderbird.net/2023/07/our-fastest-most-beautiful-release-ever-thunderbird-115-supernova-is-here/
Their flatpak is still on 102.13.0 btw. I assume they recommend the flatpak because they maintain it, thus are in control of when it gets updated. Most distros just update to the latest major release soon as its available which they clearly are not expecting.