Stand-alone boot code Cortex M3

I have a simple Eclipse ARM GNU project LED blinker running on a custom STM32F100C8 board
When I build the project and run on the board via a Segger JLink, the LED blinks fine.
If I then remove the JTAG and power cycle, the code no longer works as stand alone.

I have isolated the issue down to the startup assembly code.
If I "hard wire" the reset vector to point to main:

.syntax unified
.thumb

.section .isr_vector
Vectors:
.word __stack /* stack top address */
.word main /* Reset. First code address after reset */

the linked output is (extract):

Disassembly of section .text:

00000000 <Vectors>:
   0: 20002000 .word 0x20002000
   4: 00000141 .word 0x00000141

00000140 <main>:
 140: b480 push {r7}
 142: b083 sub sp, #12

the code works standalone after power cycle and removal of the JTAG.

However, if I use instead a vector to Reset_Handler, which subsequently BX to main (and does nothing else), the code still works with JTAG, but not standalone after power cycle:

.syntax unified
.thumb

.section .Reset_Handler
.thumb_func
Reset_Handler:
 bl main // Call application entry point

.section .isr_vector
Vectors:
.word __stack /* stack top address */
.word Reset_Handler /* Reset. First code address after reset */

Disassembly of section .text:

00000000 <Vectors>:
   0: 20002000 .word 0x20002000
   4: 00000134 .word 0x00000134

00000134 <Reset_Handler>:
 134: f000 f804 bl 140 <main>
 138: 4770 bx lr

00000140 <main>:
 140: b480 push {r7}
 142: b083 sub sp, #12
etc...

Changing the reset vector in this way is the only difference between the builds.
Link script, main.c, Makefile are all otherwise identical.

Can you suggest what the issue might be?
It seems odd that in the case that works, the linker generates a reset vector that is an odd address (and yet it still works) whereas in the case that doesn't work, the reset vector is even (which I would expect to work).

Thank you.