Not sure if this is a bug or a feature - if it is a feature, than it is a very bad one. (my opinion)
When I search a topic with many of my posts for my username, I get only 50 results.
For example here:
[RR] Round Robin im Deutschen Forum – NEU: Black&White und Portrait! Schau vorbei und mach mit 
- Language & Geographical Communities / German - Deutsch - Postcrossing Community
No chance to find my latest entry, I have to go to the last post and scroll up until I find my post which I am looking for.
Sorry if this has been addressed before, I can’t remember…
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If you are in the topic and find any of your posts, click your own name, and under the message button there is a second button “x posts in topic” (x is a number).
Clicking that button will show you a list of all your posts in that topic. I don’t think that list is limited to 50 posts.
I hope this helps I know it doesn’t exactly fix the issue.
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thank you, it has helped!
at least with this search…
it works fine with my own user and also with other users, but NOT with those whose public profile is hidden.
And it does not work when you are searching for a certain word, for example “deadline”, than again the search is limited to 50 posts. 
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I think there’s another workaround to get a more complete results list: when you open the search box and type eg, nordbaer, press enter — you’ll be taken to a search results page with the first 50 results loaded, but if you keep scrolling it will load more matching results.
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dear @Nordbaer
Yes, @paulo is right .
I can go back to Oct '20 
I also can find posts/ comments of the users with a “hidden profile”.
dear @JetteLise, yes, I also can go back to Oct. 20. But with the 50 post limit it stops at Dec. 20.
And I want to see later posts.
When I enter my username in the search field I get only 50 posts starting at Oct.20, BUT I can do an advanced search where I can raise the limit from 50 to xxx. (but I have to raise it first, only scrolling forward as Paulo wrote is not possible)
But that was not my question.
I asked for the limit for a search WITHIN a topic - when I enter my name and click on “search this topic” only 50 posts appear, and I have no option to do an advanced search here!
aaaaahhhhhhhhhh… I just found out something new - when I enter only my name as search parameter and click on “search this topic”, it is as I have written it in the first post. But when I enter the parameters “nordbaer topic:3126” it works as @paulo described it.
bit of confusing, isn’t it? 
Sorry, I could have been more clear. What I meant to emphasize that once you type what you want to search, to hit Enter so that you go to a search results page instead of seeing them inline with the topic. That way (and by scrolling down) you can see has many results as you can muster. Does that make sense?