Strange behaviour by Photoshop! I can see no obvious reason why JPEG is not offered as an option for saving.
Multiple layers in an image are automatically flattened by PS when saving in a format that doesn't support layers.
TIFF images support multiple pages and when saved in a format not offering this support, all but the first page (image) are ignored.
If you wish to experiment, here are 2 bog standard TIFFs (i.e. within PS capabilities to convert to JPEG)...
Single-page TIFF - 174 KB
Multi-page TIFF - 1.43 MB
Right-click the links and "Save Target/Link As..."