HI,
The trigger is only firing once from what I can see. Checking the logs etc. I know there may be some issues with the trigger, but it is working in most scenarios, just one specific scenario where I think something else is being triggered after.
MyField is always updating, but in this specific scenario it is reverting to previous value immediately. I'm wondering is there some sync jon or something happening after. But I don't know how to find it.

trigger ContentDocumentLinkAfterTrigger on ContentDocumentLink (after insert, after delete) {
    Set<Id> parentDoc = new Set<Id>(); 
    if (Trigger.isInsert||Trigger.isDelete) {
        List<ContentDocumentLink> cdls = Trigger.isInsert ? Trigger.new: Trigger.old;

        for (ContentDocumentLink cdl : attchs) {
            if (cdl.LinkedEntityId.getSObjectType() == Document.SObjectType) {
                parentDoc.add(cdl.LinkedEntityId);
            }
        }
        if (!parentDoc.isEmpty()) {
            Map<Id, Integer> docAttachmentCount = new Map<Id, Integer>();
            for (ContentDocumentLink cdl: [select Id, LinkedEntityId from ContentDocumentLink where LinkedEntityId in :parentDoc]) {
                Integer count = docAttachmentCount.get(c.LinkedEntityId);
                if (count == null) count = 0;
                count++;
                docAttachmentCount.put(cdl.LinkedEntityId, count);
            }

            Map<Id, ContentDocumentLink> documentAttachment = new Map<Id, ContentDocumentLink>();
            for (ContentDocumentLink cdl: [select Id, ContentDocumentId, LinkedEntityId from ContentDocumentLink where LinkedEntityId in :parentDoc]) {
                documentAttachment.put(cdl.LinkedEntityId, cdl);
            }
            
            List<Document> docList = [
                    select Id
                    from Document
                    where Id in : parentDoc];

                for (Document doc: docList){
                    if(documentAttachmentCount.get(doc.Id)>0){
                        doc.MyField = documentAttachment.get(doc.Id).contentDocumentId;
                    }
                    else{
                        doc.MyField = null;
                    }
                }
            update docList;
        }
    }
}