Your red blood count/ red cell volume can be checked.
Vitamin B12 deficiency (as it is needed for cell separation) anemia has a low red cell count (as they can't devide well anymore, hence stay big, but low in numbers), but is really the last stage of VitB12 deficiency, so very unlikely to feel anaemic due to real red blood anemia.
Easy and cheap to check though.
You don't want VitB12 to get too low to show bad symptoms like last stage anemia.
I understand you like to know the reason for as to why. Since you have researched them. Some cannot be 'healed' as you will have found out too, and you always need to take the supplements.
If the stomach doesn't produce the intrinsic factor is one story,
if VitB12 doesn't get absorbed enough from intestine into blood stream is another.
How much your intake is, how much eaten food hinders absorbtion itself (I read once about an acid in chocolate but also paleo diet hindering absorbtion, but usually intake so high, that this restriction doesn't cause concern), or how much unavoidable pesticides in our food -thanks to us human ideots killing this planet shortsightetly for mass production- restrict the absorbtion is another.
If your VitB12 is already low, that is proof enough (other people have normal VitB12 levels and need methylmalonic acid (MMA) and homocysteine and/or holotranscobalamin tested to show a problem in this area)
Have you been checked for celiac as an absorbtion problem? (simple blood test while on gluten containing diet)
I am sure you don't smoke (as nicotin can hinder VitB12 absorbtion), you do take probiotics to support guts, don't you.
Yes inflammation of guts can be one reason, but not THE reason.
If your stomach has low acid naturally
or you take PPI, VitB12 uptake can become a problem.
Again, usually we eat so much of VitB12, that it is ok still with underlying low stomach acid or low intrinsic factor production, if not: one takes supplements additionally and needs no further investigation (eg if celiac is excluded, has no pain and person takes PPI = pretty clear what was happening)
You should have a doc listening to you. If you present him with the list for finding the reason, he might already tell you, what the reason in your case was. ?
For inflammatory bowel disease you need a set of symptoms, pain.
I don't think any physician will issue a colonoscopy if you have no symptoms at all, just VitB12 deficiancy as it is unlikely an IBS problem due to missing symptoms. Or maybe can do a calprotectin stool test and occult blood in stool test first? It comes down to indication for more invasive and expensive diagnostic tools. Colonoscopies are also done as cancer prevention in some countries, if you were over e.g. 50+ years of age. If age applies you would more likely get it without other indication.
As you said you researched tons of reasons, but you need to also research the symptoms with it.
You surely came accross many reasons, that are not IBD related, too.
I would ask my doc again, what he thinks was the reason behind the low VitB12 and go through your found list, if anything was there to prove the origin of problem and if the origin could be tackled at all or not, in case found.