@Lokhan Wong Thanks for reaching here! Yes, you are correct, We support multiple mechanisms on an SPF record but not SoftFail. SPF validation only work with HardFail -all instructions and for SoftFail ~all records not supported.
If still you want to perform the SPF verification using ~all instead of -all could you Please reach out to us by sending an email with subject line “Attn:Sneha |thread title” to AzCommunity[at]microsoft[dot]com referencing this thread, would work closer on this.