There is a bug for this specific user agents:
“Mozilla/5.0 (iPhone; CPU iPhone OS 12_3_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) GSA/79.0.259819395 Mobile/16F203 Safari/604.1”
when clicking in portfolio list on slide post type the first image doesn’t appear. Only after clicking on next it works.
can you find the issue also in your demo default theme