Packed structs unaligned access

I'll start with a code sample

typedef struct {
  uint16_t len;
  uint16_t crc;
  uint8_t data[1];
} __attribute__ ((packed)) packet_t;

typedef struct {
  uint64_t timestamp;
  uint8_t val1, val2, val3, val4;
} __attribute__ ((packed)) msg_t;

uint8_t buffer[64];

packet_t *packet = buffer;
msg_t *msg = buffer + sizeof(packet_t) - sizeof(uint8_t);
msg->timestamp = 0xAA;

We use packed structs for communication. The following code crashes on msg->timestamp line. It calls __aeabi_write8. Which looks like this

0x00025B80 B570 PUSH {r4-r6,lr}
0x00025B82 4604 MOV r4,r0
0x00025B84 460D MOV r5,r1
0x00025B86 2307 MOVS r3,#0x07
0x00025B88 7010 STRB r0,[r2,#0x00]
0x00025B8A 060E LSLS r6,r1,#24
0x00025B8C 0A00 LSRS r0,r0,#8
0x00025B8E 4330 ORRS r0,r0,r6
0x00025B90 0A09 LSRS r1,r1,#8
0x00025B92 1C52 ADDS r2,r2,#1
0x00025B94 1E5B SUBS r3,r3,#1
0x00025B96 D5F7 BPL 0x00025B88
0x00025B98 4620 MOV r0,r4
0x00025B9A 4629 MOV r1,r5
0x00025B9C BD70 POP {r4-r6,pc}

When the POP happens at the end, PC register is set to some odd location (0x14), then it starts executing some random code and crashes. I also experienced similar problem with memcpy/memset functions. Any help is appreciated.

We are using 32-bit ARM Cortex M0 CPU (This is nRF514xx series chip) and Armcc v5.04.0.49.