They're supposed to have a solid core of spruce wood but since the podzol generation parity has been 'fixed' large spruces no longer have a solid wood core but now only have an odd wood block scattered up the core when generated with bonemeal at least.
Thank you for your report!
We're actually already tracking this issue at MCPE-63658, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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