can collagen peptides cause diarrhea？

Adding a quality control perspective here: poor-quality raw materials can absolutely cause GI distress.
If the manufacturer uses cheap, harsh chemical acids during the extraction process instead of a clean enzymatic hydrolysis, chemical residues or high levels of sodium/sulfites can be left behind in the powder. Our GI tracts are very sensitive to these impurities. This is why you must demand a full Certificate of Analysis (COA) from your bulk supplier to ensure the raw material is highly purified and free of heavy metals and harsh processing aids.