Hayo,
When ingesting, Photo Mechanic will preserve the capture time of the photo as the file system's creation time. However, the file system's modification time will be set to the time that the file was ingested. This may differ from previous versions (that may have also reset the modification time) but it isn't a bug. For example, if you caption a photo after ingesting, the modification time will change to the time you captioned the photo.
The reason the file size changes on ingest is that PM will append its preferences to the end of the file in order to preserve the four digit frame number that is part of the filename (per Exif spec). Otherwise this useful piece of information would be lost if you
did rename your photos on ingest. Additionally, if you lock a photo in the camera, PM interprets this as "tagging" the photo (a useful feature we invented) and this tag is also stored in the preferences area since there is no other standard place to put this information. So, you should expect your JPEGs to increase by exactly 2060 bytes when PM ingests your JPEGs.
Additionally, if you have your PM Color Management preferences set to embed ICC profiles to camera JPEGs, then of course we will honor that setting and insert the appropriate ICC profile. This will increase the file size depending on the size of the color profile you are using. I personally don't do this because since Exif 2.21 there is now a reliable way to determine AdobeRGB profiles versus sRGB.
Other than the above, your file remains pristine.
PS - a lot of people find it very useful to embed a default IPTC template during ingest that includes at a minimum your copyright information. Of course this will modify the file but you shouldn't worry about that because there are literally millions of photos that have been processed this way without a problem.
--dennis