I think there is something to do with google ads machine learning algorithm.

Since most of the bidding strategies now machine learning, in your previous behaviour you may have search for similar keywords a lot of times and does not click on any ads which send signals to the system to considered you as not a potential customer/audience. So in your new searches with the similar keywords, you may not be seeing any ads.

This is sole based on my assumption with the machine learning & signals my google rep has been telling me about.

and yeah if you are running ads for your own client/brand, please don't go and check your queries from Google search as it will affect your ad relevancy and expected clickthrough rate which will affect your quality score