This series came in two configs just the iGPU or with a second GPU As you can see here in this listing [https://everymac.com/ultimate-mac-lookup/?search_keywords=A1398|All A1398 systems]
If an external display directly plugged in shows the bars then you’ve ruled out the internal display as well as its cable. What you are encountering is an iGPU (Intel CPU) logic failure, that’s is before the AMD GPU becomes active if present.