Afternoon sun is stressful to a gardenia, especially during the time it is trying to get roots established. Both dry and soggy wet conditions are detrimental to roots. The symptoms you plants are exhibiting point to soil moisture problems. Perhaps the root zone is staying saturated and the roots are not able to get oxygen and thus are dying. Dig down a few inches deep around the plants to feel the soil and water accordingly. If the initial damage was not too severe they should be able to recover in time.
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