Hello @Francesc_MR,
starting with your last question:
This always did happen to all of us - more or less frequently, though. The difficulty is that many local guides never realize it for a long time. Marking the rejected photos, as in your screenshot, happens only if media was taken within the EU. For the rest of the world, this happens completely silently.
Indeed in your case, you contributed 25,959 photos and 2,473 videos, thereof only 26,797 media are public. Thus, 1,635 photos or videos are hidden. Also 76 of your reviews are hidden.
All our contributions are checked by a filter and if either text of a review or the content of media is not obeying some rules or suspicious for SPAM, the contributions is hidden.
If photos or video are attached to a review and only one of them is rejected, the entire review including all other attached media will be hidden.
The filter can make mistakes and sometimes a manual check (as a result of the appeal, up to 6 weeks later) can re-instate your contributions. However, if the check reveals more problems, e.g. duplicate or redundant media, the filter might act even more rigid in the future.
I don’t know, who made this suggestion, but this absolutely wrong! Nothing will change, except the filter might mark you as spammer and get even more rigid.
I suggest reading @MortenCopenhagen’s series of articles: How to Get In and Out of Trouble: The Main post
I also gave an example recently, why text-only reviews might be rejected, but according to the number of hidden media, the latter might be the bigger problem for you.
Also very helpful is @ErmesT’s article: Is Two Always Better Than One? The Most Common Photo Violation on Google Maps