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This issue caused me some headache as well... Turns out the Intel C3000 Atom platform has support for something called Intel QuickAssist Technology (Intel QAT, https://www.intel.com/content/www/us/en/architecture-and-technology/intel-quick-assist-technology-overview.html). The kernel driver for this feature is what is causing the issue with LUKS/dm-crypt. The QAT kernel driver takes precedence when both AES-NI and QAT is activated so the solution is to either blacklist the kernel module or disabling the QAT feature in BIOS. I have a Super Micro board and the setting for QAT was found under the south bridge configuration (IQAT). As a bonus the performance with AES-NI compared to QAT was twice as fast (according to cryptsetup benchmark)